The regenerative cycle executed by the machine described in U.S. Pat. No. 4,622,813, issued to Matthew P. Mitchell, co-inventor of the invention claimed herein, has come to be known as the "Sibling Cycle". That cycle may be employed in engines in which heat energy is converted into mechanical energy or in refrigerators/heat pumps in which mechanical energy is put into the machine and heat is transferred from one part of the machine to another. The machine described in that patent has theoretical advantages over other machines that employ variations of the Stirling and Ericsson cycles both in terms of mechanical simplicity and in the manner in which thermodynamic processes are carried out.
In the Sibling Cycle, two or more volumes of working gas are each sequentially subjected to the basic steps of a Stirling or Ericsson cycle. That is, first the working gas is compressed in a compression chamber and heat generated by that compression is removed from the compressed gas and rejected from the machine. The working gas is allowed to expand into an expansion chamber where it cools, and the cool, expanded gas absorbs heat. The working gas is then returned to the compression chamber to repeat the cycle. Typically, the working gas passes through a regenerator in one direction in one part of the cycle and in the other direction in another part of the cycle. This is true of the Sibling Cycle as well.
To accomplish the Sibling Cycle, the machine described in U.S. Pat. No. 4,622,813 requires at least four valves, the operation of which must be synchronized with the piston motion. Ordinary types of valves require complex mechanical or electrical arrangements to open and close them at the proper times. If the valves are separately actuated, then there is a possibility of independent failure of one valve, with possibly serious consequences. The valves must provide a good seal against pressure from first one direction and then the other. Sealing a rapidly opening and closing bi-directional valve is a difficult technical problem.
High-performance Stirling cycle engines have normally employed external heat exchangers in addition to regenerators in order to absorb heat and reject heat. External heat exchangers are expensive and add bulk and weight. Since the previously-known Sibling Cycle machine requires at least two sets of heat exchangers, each controlled at each end by valves, the burdens of cost, bulk and weight are increased if the usual type of external heat exchangers is employed.
Crank-driven Sibling, Stirling and Ericsson machines require crankshafts, connecting rods, cross-heads and piston rods. Free-piston Stirling machines do not require these moving parts, but do require separate pistons and displacers, together with gas spring bounce spaces to accommodate the motion of the pistons. These machines require at least one piston and one displacer to accomplish their thermodynamic cycle. The gas-spring bounce space generates irreversible heat transfers, reducing efficiency.